Within the scope of this specification, liquids are understood to be low- and high-viscosity liquids of all types, and in particular also liquids whose viscosity is a function of the temperature to a high degree.
Installations of the type mentioned at the outset, by means of which liquids can be conveyed from a container to one or several dispensing stations, are for example employed in machines for the application of adhesives. Adhesives, as well as other pastes, such a dyes, products used in the food industry and materials processed in the chemical industry, which have comparatively high viscosities, are often processed at temperatures which differ from the ambient temperature. They are conveyed via conduit systems with rigid and/or flexible conduits, which can be cooled or heated, to dispensing heads and are dispensed by the latter to a carrier material at the dispensing stations.
For example, in many cases it is necessary to dispense continuous ribbons of liquid to the carrier material, which is in an endless form or in the form of directly successive material sections and is conducted past the dispensing heads. In other cases there is the requirement of providing several application locations of the same carrier material with the same liquid wherein, depending on the occasion, either continuous liquid ribbons or liquid beads limited in length are to be created.
To generate pressure in the conduit systems it is possible to employ intermittently operating pumps, such as piston pumps, or continuously operating, or respectively rotating, pumps, such as gear pumps.
Gear pumps convey in a volumetric manner and are therefore mainly suitable for the chronologically constant dispensation of liquids, for example of continuous liquid ribbons on carrier materials. If a carrier material is to be provided with a liquid only in sections, the associated dispensing head must release the liquid intermittently, so that the dispensed liquid forms a plurality of liquid beads, which are spaced apart and whose length is limited, instead of an endless ribbon of liquid. An increased amount of liquid is dispensed at the start of each dispensation period because of the sluggishness of the installations and of the properties of the gear pumps, which results in that the liquid beads being generated have an excess of liquid at the start, which is unwanted in many cases.
In contrast to volumetrically conveying gear pumps, double-acting reversible piston pumps are purely pressure-generating systems. In the course of dispensing a liquid, the piston moves in such a way that the pressure drop occurring because of the dispensation of liquid is always compensated. As long as the associated installation does not dispense liquid, the piston does not move and the piston pump does not convey liquid. It is obvious that piston pumps are particularly suited to the intermittent dispensing of liquids, because they always dispense chronologically limited amounts of liquid during the respective dispensing periods, so that the liquid beads being created in the process do not have increased starting sections, but instead are even over their lengths. Neither the length of the dispensing periods nor their chronological spacing have any effects on this behavior. On the other hand, because of their intermittent operation, installations with a piston pump are not suitable for creating continuous ribbons of liquids.